Sound is composed of 'waves' that vibrate the air at varying frequencies. At certain frequencies these 'waves' becoming perceptible to the ear and are interpreted by the brain as what we call sound. Sound is able to travel through any state, liquid, gas as well as solids. In liquids and gases sound waves are longitudinal and through solids are transverse waves. Certain creatures/beings ears are attuned to different ranges of frequencies, deeming them unable to hear sounds outside of these specific frequency ranges. The human ear can monitor sounds between the ranges of 20 hz - 20,000 hz. (hz= Hertz, the measurement of sound frequencies. And yes, I wiki'd the human frequency range xD)
Sound Release in Naruto is a gift that ninja can possess which allows them to create the aforementioned soundwaves through the use of chakra, being able to freely manipulate the pitches and frequencies that the sound is released at. By manipulating these waves in the air users are able to create sounds of certain pitches, but are also able to use the vibrations in the air as a physical force, giving the waves a substance as they travel through the air. As much as I'm not sure, I'd assume due to the rapid vibrating nature of sound and it's sheer speed of movement that it is made up of the Wind and Lightning style releases. As far as it's weaknesses go, again I'm not sure. I'd say wind is a weakness of sound as it can change the speed or what not of the vibrations in the air, stopping the sound waves completely as it's saps momentum. The sheer mass of earth too would be effective as it's density would make it hard for sound to travel through, although it is known that sound can travel through any substance, it is only in it's transverse form that it travels through solids; therefore, unless the sound is created directly for this purpose Earth should stop it with relative ease.
It was introduced in the Chuunin Exam arc I believe through the use of Team Dosu, two of the three members of the team had however had their bodies modified in some way to enhance their abilities with the release. These were: Dosu, a man whom had his arm modified to produce sound waves that affected the inner ear of the opponent, damaging them from the inside. Kin, a woman whom used sound based genjutsu in order to set up further attacks, her genjutsu were initiated by a set of bells she had that would ring at a precise pitch to effect the brain. Zaku, a man whom had hollow tubes inserted within his arms that released sound infused with air pressure, the pressure of which was controlled by his chakra. His sound was given a more physical substance and was used to impact the opponent physically rather than mentally.
The only member of the sound five to actually utilize sound based attacks was Tayuya (whom as you know I will be using in the not too distant future.), Tayuya's method of using the release was through the use of a flute. By playing the flute in different melodies and at different pitches she could change the effects it had. These ranged from Sound based genjutsu to controlling her Doki (Giant ogre-like creatures attuned to the flute, different notes move different parts of their body). I believe she is the last sound user.